1. Field of the Invention
This invention relates generally to methods and apparatus for the measurement of tension in elastic surfaces which are being held in a stretched state, and more particularly to the measurement of tension in thin, plane surfaces such as may be composed of the strings of sporting rackets, continuous plastic or paper sheet, or thin stretched, flexible surfaces of various composition.
2. Brief Description of the Prior Art
The present invention is concerned with surfaces composed of either continuous or discontinuous material which has been placed under tension. Such surfaces are employed in a vast array of settings in the consumer and industrial world. In many cases it is desirable and/or necessary to have knowledge of the tension under which the surface has been placed. The present invention was devised to provide a simple, portable, hand-held method of making such a measurement, and which is also non-invasive, i.e., not requiring the surface to be broken.
A common example in the consumer sector is the stringed surface of sporting rackets. This includes tennis, racketball, squash and badminton as the more commonly known sports employing such rackets. Knowledge of the tension of the strings in these rackets is important to the successful employment of the racket in competition. No simple, reliable and portable method is presently known to exist which allows the competitor to measure the tension of the racket. Players presently rely of subjective determination by "plucking" the strings and listening to the frequency of the resulting string vibration. Some devices have been made with attempt to measure the tension from such frequency, but these have been of limited accuracy and usefulness.
Examples in the industrial sector include the need for tension measurement of continuous sheets of plastic, either during the manufacture of the plastic or during employment for packaging. The tension of paper during production or employment, as in printing operations, is also measured.
Individuals skilled in specific arts related to consumer and industrial operations will identify many other instances wherein the measurement of surface tension is desired or required.
Existing surface tension measurement systems are contained for example in U.S. Pat. No. 3,739,633 to Saxl. The device presented in that patent is based upon squeezing the material between fixed and flexible members and measuring the force on the flexible member. The device must be used near an edge and must fit on both sides of the surface. The device and method are most suited to rolling stock measurement, such as magnetic tape, and would be unsuitable for the strings of sporting rackets.
Other tension measurement systems such as U.S. Pat. No. 4,821,583 to Richards and U.S. Pat. No. 4,759,226 to Leurer are specific to the measurement of tension in thread or lines of any composition. The principles of these systems depend upon thread stock producing a force on a force resistance element, such as a spring plate or cantilever beam. These systems could not be employed for the measurement of surface tensions as for example in the mesh structure of rackets.
The present invention is distinguished over the prior art in general, and these patents in particular by a method and apparatus for the measurement of tension in a stretched elastic surface which utilizes a rigid housing having at least two diametrically opposed sides and a spring device contained therein with a contact member connected to the spring device which protrudes a predetermined distance outwardly beyond the ends of the housing sides when the spring device is an a resting state. Measuring and display devices are connected to the spring device for measuring the spring deflection when the housing is pressed against an elastic surface. The housing is placed adjacent the elastic surface to be measured with the contact member in contact with the elastic surface such that a fixed distance exists between the ends of the housing sides and the surface prior to application of a pressing force. The housing is then pressed against the elastic surface until the ends of the housing sides come into contact with the surface such that the spring device is deflected. The deflection is measured and displayed as an indication of the tension in the elastic surface. In one embodiment the spring device is a cantilever beam with the measuring device being at least one strain gauge mounted thereon. In another embodiment, the spring device is a coil spring with the measuring device being a linear variable differential transformer.